It really depends on what you are planning on storing. Here are the limitations of SQL Express. SQL Express
4GB Database Size Limit
1GB RAM Usage
1 Processor Usage
Unlike MSDE, the limiations are concrete versus not knowing where the limits are. We use SQL Express in 60-70% of our medical installations and only upgrade them to a full version of SQL when they have more data and needs for integrated imaging. Our medical imaging product allows the scanned documents to be stored within the database (integrated) or in a folder based schema. Obviously data-integrated will grow the size of the database VERY quickly, but if they choose to use folder based and security is not as big of a risk in that environment, then they can use the folder based and have only the header record, etc. in the SQL table and the media files store on disk. So you can work your application around the limiations as well. I know of some other teams that will have concrete size limitations on their databases and they use a different database for all of their media. So they will have 2-3 databases on the SQL Express that are used within their application.